This invention relates to push-push oscillator circuits for operation in the range of 20 to 40 GHZ and more particular to push-push dielectric resonator oscillators.
In the past, a variety of microwave components which employ dielectric resonators have been developed due to the availability of low cost temperature stable, high permittivity materials. These high permittivity materials are available from several manufacturers with a variety of dielectric constants and temperature coefficients. Their use maximizes the performance to size ratio for many filters and oscillators, thus enabling low cost and manufacturing components to be realized. They are therefore the logical choice for many fixed frequency receiver/transmitter low noise oscillator applications.
Dielectric resonator oscillators can generally be classified as being either reflection or feedback type oscillators. Reflection type oscillators which couple a dielectric resonator to the output circuit one half wavelength away from the transistor (such as a field effect transistor), exhibit significantly improved FM noise performance and frequency stability over that of a conventional oscillator. Reflection oscillators, however, do not achieve the low FM noise performance and high stability characteristics of feedback dielectric resonator oscillator circuits since optimum load circuit Q is not obtained. This condition exists because the gate/source circuit which dominates the frequency stability characteristic is constructed using low Q elements and is not strongly influenced by the presence of the resonator at the device output.
An alternate circuit which yields high stability and low FM noise is a series feedback oscillator such as that shown in U.S. Pat. No. 4,539,530. The series feedback oscillator circuit usually consists of a high gain, low noise field effect transistor. A terminated 50 ohm microstrip transmission line connected to the field effect transistor's gate. A coupled dielectric resonator. A shunt reactance connected to the field effect transistor source, and an impedance transfomer such as a transmission line connected to the drain port.
Critical to the performamce of the series feedback oscillator circuits is the placement of the dielectric resonator on the gate circuit of the field effect transistor where it is isolated from the output through a very low drain to gate capacitance inherent in field effect transistors. This isolation minimizes interaction between the oscillator's output and input circuits resulting in a vey high loaded circuit Q.
However, at frequencies above 20 GHz it is nearly impossible to effectively manufacture accurate, high Q resonators. Physical handling is also a major problem since a 20 GHz resonator is in the order of 1 mm in diameter. A completely different set of difficulties arises during the design of a Ka-band dielectric resonator oscillator. Since the gain of even the best field effect transistors currently available is marginal at frequencies between 26 and 40 GHz, excessive resonator coupling is required and this thus destroys the inherent Q and spectral purity of the oscillator.